I was not able to find a Catholic saint for today, just an Orthodox one. To make matters worse, the Catholic Church has declared some of his teachings heretical. One might think it foolhardy to tell his story in an east-and-west sort of setting like this. But you know me.
John Cassian (ca. 360–435), hailing either from Gaul or Scythia (the latter being somewhere between Bulgaria and Romania), traveled with a buddy named Germanus to the Holy Land, where they entered an eremitic community near Bethlehem. Unsatisfied with the life there (or the food?), they traveled to Scetis in Egypt to learn a deeper monkosity. Cassian’s experiences there led to his two greatest works, the Institutes, and the Conferences. The former had a direct influence on (St.) Benedict (Mar 14) and his great Rule, the famous set of foundational guidelines that became the guiding foundation of all western monasticism. The latter got him in trouble with the Augustinians in Rome for semi-pelagianism, but more on that anon. From Egypt Cassian and Romanus legged it to Constantinople, where they fell in with John Chrysostom (Sep 13), who deaconified Cassian. When shortly thereafter Chrysostom was exiled, Cassian went to Rome to try to get the Pope to come to Golden Mouth’s aid. Romanus completely disappears from the narrative at this point, but we’ll assume he found something wholesome to occupy his time. At any rate, Cassian was tapped to start a monastery in Gaul, where he started two just to be safe; or to be more precise, a double monastery*. He died in Marseilles, but that can be said for a lot of people who were otherwise good, upstanding Christians, so I at least will not hold it against him.
Theologically-wise-speaking, Cassian argued against Augustine’s “irresistible grace” soteriology, arguing instead that salvation, although wholly dependent upon God’s grace, also requires the cooperation of human free will. Both/and, not either/or, if you will—kind of analogous to the Incarnation, in which Christ is both fully God and fully man. For this he is considered fully orthodox in the Orthodox world, but was in the west deemed guilty of semi-pelagianism, which was declared heretical by the Council of Orange. The Orthodox position on this is relegated to the footnotes in the Wikipedia article, giving you some idea of the hardship we Orthodox toil under. (Cue: “Gloom, Despair, and Agony on Me.”) The Council however declared that as a person, he was holy enough, and most of my western sources list him as a saint and assign his feast to July 23.
As to why his day falls on Leap Day, and thus officially comes but once every four years, the Russians tell this story: Once upon a time St. Nicholas (Dec 9) and St. John Cassian were walking across Russia on their way to a conference with God. (I know; I know.) Along the way they came upon a man whose cart was stuck in the mud. He begged for their help, so Nicholas rolled up his sleeves and helped push the cart out of the mud, while John Cassian stood apart. After Nicholas rejoined him, Cassian excoriated him, saying there isn’t enough time to go to the dry cleaners, and thus he (Nick) will have to stand before God in muddy robes. When they came before the Throne, God asked Cassian why his robes were so clean, and upbraided him for not helping the man in the mud. In punishment, his feast day was set to February 29, which (as you are no doubt familiar) comes only once every four years. Although in non-leap years John Cassian’s feast is usually observed on February 28, at least nowadays, by softies.
I have heard that some people think this tale is apocryphal. I was unable to verify their existence, however.